1. Field of the Invention
The invention relates to an Anterior Cruciate Ligament (ACL) prosthesis and an anchor to fix its ends into bones. More particularly, it relates to an ACL prosthesis formed of a plurality of independent filaments and an anchor for minimizing wear and failure.
2. Discussion of Related Art
Anterior Cruciate Ligament (ACL) in the human knee joint, commonly called Cranial Cruciate Ligament in the canine stifle, is frequently torn in trauma, or, as it happens in dogs, fails after a degenerative process of still unknown etiology. Direct repair is usually not possible—when attempted, it predictably fails.
In human orthopedics, the standard procedure calls for replacement by an allograft, a part of the patellar ligament, or a part of the tendon removed from hamstring muscles. The procedure results in a stable knee, but the long term performance is often unsatisfactory with over a half of cases resulting in arthrosis of the joint.
In dogs the standard procedure is either an extracapsular suture (usually placed on the lateral side of the joint) approximating the function of the ligament, or one of the geometry modifying surgical techniques, e.g. Tibial Plateau Leveling Osteotomy (TPLO), Cranial Closing Wedge Osteotomy (CWO), or Tibial Tuberosity Advancement (TTA). Intra-articular prostheses are also occasionally used, but those generally end up in failure. Extracapsular sutures also fail. These are intended to provide stability of the joint for several weeks while waiting for fibrosis to form around the joint, which then should provide for long term stability. Arthrosis of the joint at longer term, at a year or so, is the rule rather exception.
There are a variety of anchors used to fix the ends of an ACL prosthesis into bones. Most commonly, they are so-called interference screws, designed to be inserted along the prosthesis (transplanted tendon or ligament, or an artificial ligament) within an anchor hole, or tunnel, drilled in the bone. The interference screw jams the prosthetic tissue against the bone within the anchor hole. FIG. 1 shows an ACL bone anchor 1 of the interference screw type inserted into bone 3 in the direction 5 so as to jam the prosthetic tissue 2 within the hole 4 which can then resist pull-out force 6. Such screws are made either from metal, most commonly titanium, or bioresorbable polymers.
Another common technique is so-called cross-pin used to anchor a loop of the prosthetic tissue within a hole drilled in the femoral condyle. In all cases, prosthetic tissue exits the tunnel by bending over the edge of the bone; healing/remodeling of the bone is expected to fill the gaps and to result in a natural-like anchorage of the ligament in the bone. FIG. 2 shows an ACL anchor 7 of the transverse, or cross pin type, inserted into the bone of the femoral condyle 12 through a hole 10. A front section 8 of the anchor 7 passes through a loop of prosthetic tissue 9 inserted through hole 11. In this manner the prosthetic tissue can support the pull 13.
Neither of these techniques is suitable for a permanent anchorage of an artificial ACL replacement. Bending of the prosthesis over the edge of the hole will lead to both, bone loss due to contact resorption and mechanical damage—ultimately failure—of the prosthesis due to bending and wear at the edge of the hole.
An alternative method of using screw anchors with the prosthesis fixed in their central, axial hole also do not provide for a durable anchorage, because in all known designs the bending of the prosthesis over the edge of the hole exceeds the fatigue limit of even the best materials used for prosthesis.